Balances of this kind are used in the laboratory where, as a frequent occurrence, substances are filled into a container that is placed on a weighing pan or on a weighing object carrier of a balance. In the laboratory, filling containers with materials to be weighed is done mostly by hand. Pulverous or liquid weighing material is taken out of a supply container by a spatula or a spoon and—with the draft shield of the balance opened up—delivered into a weighing receptacle which rests on the weighing object carrier with the opening of the receptacle facing upwards. This involves the risk that weighing material is spilled on its way to the weighing receptacle, that it falls on the weighing object carrier and is inadvertently included in the weighing. Furthermore, the hand of the user enters into the weighing compartment and causes a warming of the air surrounding the weighing receptacle. Added to this is the problem of the air draft when at least one wall panel of the draft shield is completely opened in order to allow access to the weighing receptacle. Taking a substance to be dispensed out of a supply container that is placed outside of the weighing compartment can involve a time-consuming manipulation as it often involves multiple passes through an opened wall panel of a draft shield with which a balance is normally equipped. This leads to further air movements which can disturb the thermal equilibrium in the weighing compartment. Experience shows that after the weighing receptacle has been filled and the draft shield has been closed, it will take some time until the atmospheric conditions around the weighing receptacle are stabilized and a reliable weighing result can be obtained in the subsequent weighing of the substance that has been filled into the receptacle.
In laboratories of the pharmaceutical and chemical industries, the weighing of the smallest amounts, primarily in the form of pulverous substances, is of increasing importance. The substances are often expensive or even toxic. In applications that are subject to regulations and where error tolerances have to be strictly complied with, the ability to reliably measure even the smallest sample quantities is of the highest order of importance. As a result, the time required to reach stable weighing conditions after the filling process can thus easily extend beyond a few seconds and take several minutes, which runs counter to an efficient work flow.
As an alternative, the receptacle could also be filled outside of the balance. For this purpose, the weighing material is weighed on a weighing paper or in a weighing boat and subsequently transferred to the receptacle. This procedure is less desirable as the user cannot be certain of having transferred all of the substance, so that it could again turn out to be better to perform the filling directly into a weighing receptacle that is set in place on the weighing pan inside the weighing compartment.
A balance which is described in U.S. Pat. No. 6,603,081 B2 has a weighing compartment, a balance housing containing the weighing cell and forming the rear wall and the floor of the weighing compartment, and a carrier device which serves to receive the weighing object and is attached to a cantilevered extension of the weighing cell. A fixed holder system is attached to the rear wall of the weighing compartment, which allows weighing compartment accessories to be fastened at different heights above the floor of the weighing compartment. A holder for laboratory devices or an intermediate floor can be attached to the holder system, where the intermediate floor can be useful as a place on which to set substance containers, tools, etc. The supply container for the substance to be filled into a receptacle can now be placed inside the weighing compartment, e.g., on the intermediate floor. However, the spatial situation inside the weighing compartment can involve especially careful hand movements so as to avoid knocking over the supply container that has been put on the intermediate floor. Furthermore, the air in the weighing compartment can still warm up too much if the hand of the user dwells in the weighing compartment too long.
A further unsolved problem concerns the flexibility of a balance and the conditions that the balance imposes on the weighing receptacles that the weighing substance is filled into. The analyses or reactions that follow after the weighing process involve the broadest diversity of weighing receptacles of different sizes, such as a long-necked measuring flask, an Erlenmeyer flask, or test tubes.
In a balance which is described in DE 87 16 977 U1, the problem of avoiding internal air currents inside a closed draft shield is addressed by keeping the height of the draft shield as low as possible and choosing a design where in the case of taller weighing receptacles the height of the draft shield can be expanded in such a way that an accessory enclosure top can be placed on the topside of the draft shield and secured by engaging the guide tracks of the top panel. A special version of this accessory enclosure top has a closable access opening at the topside to allow the filling of a measuring flask standing on the weighing pan.
It has been found to be a drawback, particularly when filling pulverous substances into a narrow-necked weighing receptacle placed on a balance according to DE 87 16 977 U1, that the filling from above is extremely undesirable from an ergonomics point of view and therefore creates the risk that the substance may get spilled.